qtnfmac: switch to 32bit values for RTS/FRAG thresholds

Host wireless stack uses u32 type for RTS/FRAG threshold values.
Switch to u32 in driver: pass u32 values to firmware and let
firmware properly adapt these values according to its
internal representation.

Signed-off-by: Sergey Matyukevich <sergey.matyukevich.os@quantenna.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>